Regex literal syntax (/pattern/) in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a regex literal in Ruby?

A regex literal in Ruby is a pattern written between slashes, like /pattern/, used to match text.

beginner

How do you write a regex literal to match the word 'cat' exactly?

You write /cat/. This matches the exact sequence of characters 'cat' anywhere in the text.

intermediate

What does the regex literal /\d+/ match?

It matches one or more digits in a row. \d means any digit, and + means one or more times.

intermediate

How do you include options like case-insensitive matching in a regex literal?

Add a letter after the closing slash. For example, /cat/i matches 'cat', 'Cat', or 'CAT'.

intermediate

How do you write a regex literal to match a string that starts with 'hello'?

Use the caret symbol ^ at the start: /^hello/. This matches 'hello' only at the beginning of the text.

What does the regex literal /ruby/ match?

A The exact sequence 'ruby' anywhere in the text

B Only the word 'Ruby' with capital R

C Any word ending with 'ruby'

D Only the start of a string

The regex literal /ruby/ matches the exact sequence 'ruby' anywhere in the text, case-sensitive.

How do you write a regex literal to match one or more digits?

A /\d+/

B /\w+/

C /\s+/

D /\D+/

\d matches digits, and + means one or more times.

What does the i after a regex literal do? For example, /cat/i

A Matches only at the start of the string

B Makes the regex match whitespace

C Matches only digits

D Makes the match case-insensitive

The i flag makes the regex ignore case differences.

Which regex literal matches a string that starts with 'hello'?

A /hello$/

B /^hello/

C /hello/

D /^hello$/

The caret ^ means the start of the string.

How do you write a regex literal to match a newline character?

A /\s/

B /\t/

C /\n/

D /\r/

\n matches a newline character.

Explain how to write a regex literal in Ruby and how to add options like case-insensitivity.

Think about the slashes and letters after them.

Describe how anchors like ^ and $ work inside regex literals.

Anchors fix where the match happens.

Practice

1. What is the correct way to write a regex literal in Ruby to match the word cat?

easy

A. regex('cat')

B. "cat"

C. 'cat'

D. /cat/

Solution

1. Step 1: Understand regex literal syntax

   In Ruby, regex literals are written between slashes, like /pattern/.

2. Step 2: Identify the correct pattern for 'cat'

   The pattern to match the word 'cat' is simply /cat/.

3. Final Answer:

   /cat/ -> Option D

4. Quick Check:

   Regex literal = /pattern/ [OK]

Hint: Regex literals always use slashes /pattern/ in Ruby [OK]

Common Mistakes:

• Using quotes instead of slashes
• Calling a non-existent regex() function
• Confusing string and regex syntax

2. Which of the following is the correct regex literal to match the word Dog case-insensitively in Ruby?

easy

A. /Dog/m

B. /Dog/g

C. /Dog/i

D. /Dog/

Solution

1. Step 1: Identify the case-insensitive option

   In Ruby regex, the i option after the slashes makes matching case-insensitive.

2. Step 2: Check the options given

   Only /Dog/i uses the correct i flag for case-insensitive matching.

3. Final Answer:

   /Dog/i -> Option C

4. Quick Check:

   Case-insensitive flag = i [OK]

Hint: Use /pattern/i for case-insensitive regex in Ruby [OK]

Common Mistakes:

• Using g or m flags which mean different things
• Omitting the i flag for case-insensitivity
• Placing flags inside the slashes

3. What will be the output of this Ruby code?

text = "Hello World"
match = text =~ /world/i
puts match

medium

A. nil

B. 6

C. 0

D. Error

Solution

1. Step 1: Understand the =~ operator with regex

   The operator =~ returns the index of the first match or nil if no match.

2. Step 2: Analyze the regex and string

   The regex /world/i matches 'World' ignoring case. In "Hello World", 'World' starts at index 6 (0-based).

3. Final Answer:

   6 -> Option B

4. Quick Check:

   Case-insensitive match index = 6 [OK]

Hint: =~ returns match start index or nil if no match [OK]

Common Mistakes:

• Ignoring case-insensitive flag i
• Thinking output is true/false instead of index
• Assuming match starts at 0 always

4. Identify the error in this Ruby regex literal:

pattern = /hello/i/

medium

A. Extra slash at the end causes syntax error

B. Missing quotes around pattern

C. Missing i flag for case-insensitivity

D. Regex literals cannot have flags

Solution

1. Step 1: Check regex literal syntax

   A regex literal starts and ends with a single slash. Here, there is an extra slash at the end.

2. Step 2: Identify the syntax error

   The extra slash after /hello/i is invalid and causes a syntax error.

3. Final Answer:

   Extra slash at the end causes syntax error -> Option A

4. Quick Check:

   Regex literal must start and end with one slash pair [OK]

Hint: Regex literals have exactly two slashes, no extras [OK]

Common Mistakes:

• Adding extra slash after regex
• Confusing regex with string quotes
• Thinking flags go inside slashes

5. You want to find all words starting with a or A in a string using a regex literal. Which regex literal correctly matches this requirement?

hard

A. /\b[a]\w*/i

B. /\b[A]\w*/

C. /\b[a]\w*/

D. /\b[a-z]\w*/i

Solution

1. Step 1: Understand the requirement

   We want words starting with 'a' or 'A'. Using case-insensitive flag i simplifies matching both.

2. Step 2: Analyze options

   /\b[a]\w*/i uses /\b[a]\w*/i which matches word boundary, then 'a' (case-insensitive), then zero or more word chars. This matches words starting with 'a' or 'A'.

3. Final Answer:

   /\b[a]\w*/i -> Option A

4. Quick Check:

   Use i flag to match both 'a' and 'A' easily [OK]

Hint: Use /pattern/i to match letters ignoring case [OK]

Common Mistakes:

• Listing both 'a' and 'A' inside brackets without i flag
• Using wrong character classes
• Missing word boundary \b